Manufacture of silicone elastomers with the aid of a silent discharge



emmms United States Patent F MANUFACTURE OF SILICONE ELASTOMERS WITH THE AID OF A SILENT DISCHARGE Frederick L. Thomas and Francis F. Koblitz, Madison, Wis., assignors to Bjorksten Research Laboratories, Inc., a corporation of Illinois N Drawing. Application July 19, 1952, Serial N0- 299,914

3 Claims. c1. 204-165) This invention relates to the production of transparent silicone elastomers and to methods of further strengthening and cross-linking them.

Heretofore siloxane elastomers have been made which Nowhere has such a silicone rubber been made which is fully transparent or which could be used as an interlayer material for aircraft glazing components. Transparent silicones exist which are flexible but which are not of sufficient strength and elasticity.

Under the existing art dialkyl or diaryl silanes, or combinations of the two, of the structure:

(which are cyclic if x is from 3 to 8 and both Rs are methyl) or the analogous di-hydroxy monomer with both Rs being aryl are opened up in the former case and condensed in the latter by means of strong mineral acids or alkalies or iron chloride to long chain elastomers. These may be cured by means of organic peroxides, lead or manganese naphthenates, lead, mercury, zinc, selenium and titanium oxides, which remain in the rubber and seriously interfere with its light transmission.

We have found that dialkyl siloxanes, and substituted dialkyl siloxanes in which the substituted groups are phenyl, vinyl, allyl, hydrogen or hydroxyl (or free radical formed when OH is split off, i. e., Si or -SiO-) can be treated by methods now to be described which appear to show improvement commonly described as cross-linking and which do not seriously interfere with the transparent quality of the silicone elastomer so as to prevent its being used as an interlayer (i. e. less than 90% light transmission in A" thick film). 1

The cyclic siloxane monomer oil or the condensed siloxane elastomer gum or any combination of these two is treated with strong hydrogen peroxide. Wetting agents may be used provided they do not form color forming residues or are removed'later. The strength of the peroxide can vary from 3% thru 99%. 30% is a practically convenient strength. Sodium peroxide can likewise be used alone or in combination with sodium hydroxide during the condensation stage of the process. Na2CO3.2H2Oz is also useful.

The oil or gum as described in the previous paragraph can be treated with a silent discharge. Such a discharge is. produced by current coming from the electrodes of a Tesla coil or from high frequency high voltage charges when the electrodes are brought into prox- 2,778,793 Patented Jan. 22, 1957 imity. The film thickness can vary from ZOO-300" up to A" but works best in the region of 5 to 20 mils. The gap between the electrode is 4;" for a 15,000 volt, 30 milliampere secondary and 115 volt A. C. primary transformer.

Ultra-violet light such as that which comes from a pressurized quartz tube-mercury type burner without the nickel oxide filter-giving spectrum from 2200 A. to 4000 A. is effective on cyclic siloxane monomer oil or condensed siloxane elastomer gum or any combination of these two. Film thickness can vary from units described previously but 20 mils to mils is a preferable range. No catalyst or dust must be present as the combination clings to the surface causing discoloration.

Chlorination in presence of near ultra-violet light with or without replacement of chlorine by hydroxyl or removal of chloromethyl groups by hydrolysis with alkali, moisture and heat is helpful. Other applicable methods for removal of chlorine or chloromethyl groups are, for example, fusion with potassium or sodium acetate to form esters and subsequent hydrolysis of esters to structures. However removal of such groups must be performed on the final polymer after it has achieved high molecular weights.

The usual cross-linking agents reactive to a primary hydroxyl group are necessary. A few basic types are listed below by way of example.

1. Chloro silanes:

(11H; CH3 Cl SIX-C1 61-81-01 OH: Ci

2. Alkoxy silanes: Na+Si(-OEt)4 With catalyst: CHsO'-Na-(removed later) 3. Dibasic acids: Oxalic through adipic acid. Acids of high molecular weight of aliphatic character enhance optical clarity where used as cross-linkers, therefore oxalic, malonic, maleic, fumaric and succinic acids are preferred.

4. Dialdehydes, such as glyoxal.

5. Glycols such as ethylene glycol although the OH group does not enhance surface wettability with siloxane or optical clarity and must be used under drastic conditions or with a wetting agent.

In the past, fillers have been incorporated into siloxane resins for the purpose of increasing the tensile strength and elasticity when curing these resins by peroxides.

Now, however, it has been found that the tensile strength of siloxane resins can be increased, at the same time preserving their transparency, by curing these resins between plates which have an inorganic filler bonded to their surface. The filler exerts its catalytic effect on the surface of thin films while not actually entering the resin itself.

The invention'is further illustrated by the following specific examples:

Example I two electrodes spaced .03" apart and charged with the;

maximum amount'of voltage which could be applied without perforation of the mass. Usually about 15,000 volts Example II 100 grams of octa methyl tetra siloxane (B. P.: 171 to 170 C.) (Ref. index: 1.3935-/D) was mixed with 1 gram of chemically pure concentrated sulfuric acid (sp. gr. 1.84) in a closed heated reaction vessel equipped with a glass stirrer capable of vigorous agitation. The

mass. was stirred and heated at 125 C. for 12 hours at i which time a sample removed and tested in Brookfield viscosimeter indicated 10,000 cps. The agitation and heat was continued until 25 hours at which time the viscosity was greater than 2,000,000 cps. The elastomer gum was removed and washed on cadmium plated screwtype rollers until free of H2804. as indicated by Ba(NOa)z di-ethyl ether, MeOH, H2O solution.

The excess water was removed in a dessicator by evacnation.

To 10 grams of dried washed gum 0.25 gram of benzoyl peroxide was mixed on above rollers. The resultant paste was spread out on aluminum sheets separated by shims and pressed 4 hours at 105 C. under hydraulic pressure, at which time the gum had become a 6 mil transparent homogeneous elastic. sheet which adhered readily to glass. The rupture strength of this sheet as obtained by clamping tightly over a A" round hole was 3 to 4 lbs. up to 6 to 7 lbs. per square inch air pressure. The corresponding strength fornthe untreated but Washed and dried gum was lessthan one pound persquare inch.

transmission was unmeasurable with an. Eastman photographic densitometer because of the thinness ofithe sheet. However, sheets of 27 to 30 mils thickness exhibited less than 0.04 optical density or greater than 90% light transmission.

Example III 10 grams of gum as described in Example II were mixed with 0.50 gram of ditertiary butyl peroxide. was pressed between aluminum sheets for 3 hours at 150 C The 6 mil films obtained were tested for rupture strength as in Example 11 and had a strength of 41 lbs. per square inch or 41 times as great as the untreated material. Optical density 33 mil film 0.06 or 87% light transmission.

Example IV 10 grams of washed, dried gum described in Example Il=were dissolved in 100 grams of CO1; and placed ina 1 liter reaction flask equipped with a gas inlet tube and ultra violet mercury are light inserted in one of the openings. Agitation was started and chlorine admitted together with 9 times its volume of dry nitrogen. After twp hours the solyentwas removed and samples analyzed fprchlorine by sodium peroxide fusion. 0.5 to.0.75% chlorine had been absorbed. The gum had a faint straw col-or, part ofthe gum was pressed into films with Grignarid. of ethylene bromide which cross-linked. it to a tougl1.n 1ass. However, the major portion wasfused'with arts'glacial aceticand parts of potassium acetate. The. gum was separated, washed free of acid, dried and heated: with, oxalic acid under slight vacuum to distill off; acetic acid. Laterit was removed and pressed-be- .li sii imf ast o 4 qur M C. A o h flex ble tranlsparnt film waslthus formed.

The mass,

Example V The 6 mil sheets prepared as in Example II were dipped in ethyl ortho silicate and hung up to dry on glass rods in a closed drying oven at 45 C. After 20 hours the strength had increased to 5.83 rupture strength, whereas the original was 4.75 lbs. The control exposed to air at room temperature, for a similar period increased in strength to 5.0 lbs/sq. inch. Optical density was unchanged.

Example VI The 6 mil sheets prepared as in Example II were. dipped in methyl triethoxy silane and hung up to dry on glass rods in a closed drying oven at 45 C. After 20 hours, the strength increased to 5.14 lbs./ sq. inch.

Example VII The 6 mil sheets prepared in Example II were dipped in a 1% solution in MeOH in Aerosol OT, a wetting agent, drained dry and dipped into a solution of 30% H202 for 2 hours. After removal and drying the rupture strength increased from 4.50 to 4.75 lbs/sq. inch. to 7.0 to 7.5 lbs/sq. inch and the optical density remained unchanged.

Example VIII The elastonier gum of Example II is pressed into thin sheets which are placed in a bell jar and chlorine gas is passed through the jar for 10 hours.

The sheets are found to have increased in strength and to be transparent.

Example IX l0,0 parts of elastomer gum of Example 11 is milled on a rubber mill for 6 hours together with 3 parts of AlCla to produce a compound having enhanced properties.

Example X The procedure of Example II iscarried out with aluminum sheets which are dipped in MgClOi prior to their use in pressing the paste.

Example X1 Example XII A, trifiuoro chloro ethylene and titanium dioxide suspension-were coatedon glassplates and then baked. 100 parts of a polysiloxane were mixed with 2.5 parts of benzoylperoxide and cured between these plates at 100 C.- for-4-honrs. The final thickness of the film was 0.006 inch.

Four; thicknesses. of this film were laminated to givea film which had greater than-% light transmission.

Example XIII parts of octamethyl tetra-siloxane are placed in a' reaction-vessel and stirred rapidly while one partof 5,111- furic acid (sp. gr, 1:86) is added. at-1'25f C for 24 hours, at which time the guru is re; moved. The. viscosity 2,000,000 centipoises.

The gum is then dissolved in petroleum ether, washedv free of free sulfuric acidby 25% methanol and water, and'dried of solvent by vacuum evacuation at 00001 mm. of mercury at roomtemperature.

100 parts. ot-the dried gum'is then mixed intimately with benzoyl;peroxideand pressed 4 hours at 100 G2 to afilm thickness; of .006 inch=betweenaluminum sheets.

The vessel is heated oi" the gum is greater than;

Four thicknesses of the cured gum are coated lightly with an adhesive and recured 1 hour at 150 C. The resulting laminated film has greater than 90% light transmission and withstands a pressure of 30 pounds of air pressure per quarter inch of surface.

The processes shown in the examples can be used to make sheets up to thick.

Example XIV A cross-linked transparent polymeric derivative of octamethyl tetra-siloxane is made by mixing about 50 parts of octamethyl tetra-siloxane with about 20 parts of diphenyl silane diol, mixing the product with about 1 part of concentrated sulphuric acid, washing, milling and drying the resultant compound and then passing a silent electric alternating current at about 15,000 volts through the resultant mass.

From the above disclosure it is apparent that the process is of wide applicability and is not to be restricted excepting by the claims, in which it is our intention to cover all novelty inherent in this invention as broadly as possible, in view of prior art. Having thus disclosed our invention, we claim:

1. A process of preparing a transparent polysiloxane resin, which comprises the step of exposing a linear transparent polydialkyl siloxane gum to a silent electrical discharge of about 15,000 volts.

2. The process of making a cross-linked transparent polymeric derivative of octa methyl tetra siloxane comprising mixing octa methyl tetra siloxane with diphenyl 6 silane diol, mixing the product with a small quantity of sulfuric acid, washing, milling and drying the resultant compound and then passing a silent electric alternating current discharge at about 15,000 volts through the resultant mass.

3. The process of making a cross-linked transparent polymeric derivative of octa methyl tetra siloxane comprising mixing about parts of octa methyl tetra siloxane with about 20 parts of diphenyl silane diol, mixing the product with about 1 part of concentrated sulphuric acid, Washing, milling and drying the resultant compound and then passing a silent electric alternating current discharge at about 15,000 volts through the resultant mass.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Rochow, An Introduction to The Chemistry of the Silicones, Wiley 1946, pg. 77.

Rochow, Chemistry of the Silicones, 2nd ed., Wiley, 1951, pg. 74. 

1. A PROCESS OF PREPARING A TRANSPARENT POLYSILOXANE RESIN, WHICH COMPRISES THE STEP OF EXPOSING A LINEAR TRANSPARENT POLYDIALKYL SILOXANE GUM TO A SILENT ELECTRICAL DISCHARGE OF ABOUT 15,000 VOLTS. 